This is an unofficial statistic used by analysts and sports broadcasters to rate how often a quarterback or a team scores a touchdown. It is usually calculated by dividing the number of touchdowns by the number of times the team was in the red zone. When applied specifically to quarterbacks, missed catches in the endzone thrown from outside the redzone will sometimes be factored into this statistic.
Most top-tier quarterbacks have a high touchdown percentage (like Tom Brady, Aaron Rodgers, or Drew Brees). Scoring is obviously essential to winning the game, so a team's overall touchdown percentage is a good indicator of their ability to score when close to the endzone. This measure is similar to scoring efficiency, but only concentrates on touchdowns, rather than including field goals. Since a touchdown is worth more than a field goal, and teams tend to score more of them, this statistic is often mistaken for scoring efficiency.